Ben Johnson: The iShares Core Total US Dollar Bond Market ETF--the ticker for that is IUSB--was launched in June of this year as part of iShares' expansion of their iShares Core series. Investors can think of this as sort of a core-plus-more exposure.

The iShares Core Aggregate Bond ETF (AGG) and the Vanguard Total Bond Market ETF (BND) both track the [Barclays U.S. Aggregate Bond Index], which offers exposure just to the investment-grade universe of the U.S. bond market. Now, this fund's benchmark--the Barclays U.S. Universal Index--includes, in addition to the investment-grade universe, high-yield bonds, emerging-markets bonds, U.S.-dollar-denominated ones, and other non-investment-grade issues.

Now, these bonds in aggregate represent about 16% of the U.S. investable bond market. So, by introducing them into the mix, investors get a portfolio that is actually more representative of the U.S. bond market than they would by buying a fund that tracks the [Barclays U.S. Aggregate Bond Index] like AGG or BND.

It's important to note that introducing non-investment-grade issues alters the return and the risk characteristics of the portfolio. Specifically, if you look at the current yield to maturity for IUSB's portfolio, it's about 2.63% versus about 2.27% for AGG's portfolio. Interest-rate risk between the two, as measured by duration, is fairly comparable. We're looking at a level of about 5.21 for IUSB versus 5.29 for AGG. Now, the greater yield, which I mentioned before, reflects greater credit risk within IUSB's portfolio relative to a portfolio like AGG's or BND's that tracks the BarCap Agg. The average credit rating of issues in IUSB's portfolio is BBB versus A for AGG.

It's also important to keep fees in mind. At 15 basis points or 0.15%, IUSB's annual fee is a bit richer than the 0.08% or 8 basis points levied by AGG and BND, but this is still a tiny fraction of the average fee levied by its peers.

All in all, I would say that this is an interesting option for investors looking for a broadly diversified low-cost option for a core fixed-income holding.